1. Lecture # 12&13 SWITCHING-MODE POWER SUPPLIES

2. Switching-Mode Power Supplies

3. Introduction to Switching Regulators
Objective of topic is to answer the following questions:
What is a switching power supply?
What types of switchers are available?
Why is a switcher needed?
How does a switcher operate in general?
How does a buck converter operate?
How does a boost converter operate?
How does a buck-boost converter operate?
How many design topologies of a switching power supplies?

4. Introduction to Switching Regulators
Intended Audience:
Electrical engineers with limited power supply background
A simple, functional understanding of inductors and capacitors is assumed
A simple, functional understanding of transistors is assumed
Expected Time:
Approximately 120 minutes

5. Outline of the presentation
Switching Regulator Overview
What is a Switching Regulator?
Why is a switcher needed?
What are the main differences between a switching and linear regulator?
Buck, Boost, Buck-Boost (Inverting)
Five designing topologies..
Flyback converter.
Forward converter.
Push-pull converter.
Half Bridge converter.
Full-Bridge converter.
3. Switching Regulator Operation .
How does a Switching Regulator works?
Switching Regulators components.
Switching mode DC power supplies

6. What is Switching Power Supply?

7. What is Switching Power Supply?
The advantage of a switching-mode power supply is that the relatively high frequency oscillator allows the use of small, lightweight and low-cost transformers.
This makes them considerably smaller and lighter than linear power supplies. Almost all modern powers supplies, including those in PCs, are switching mode power supplies. Their disadvantages are complexity and RF egress (interference).

8. What is a Switching Regulator?
Converts an input voltage into desire output voltage.
The power transistor operates as a switch, completely on or off.
An energy storage part (inductor) is used in the architecture

9. Choosing Between Linear and Switching Regulators
When possible, most designers would prefer to use a linear voltage regulator rather than a switching voltage regulator
Linear regulators are usually lower in price
Linear regulators are usually simpler to implement
Linear regulators do not have associated noise/ripple problems apparent in switching regulators
So, when should you use a switching voltage regulator instead of a linear voltage regulator?
Well, in the simples terms, most designers will use a linear voltage regulator wheneve it is possible for all the above reasons.
The linear voltage regulators are easier and cheaper to use. It is hard to beat that kind of combination.
However, there are instances when a switching regulator is the right choice.

10. Choosing Between Linear and Switching Regulators
When to use a switching regulator :
When the minimum input voltage is at or below the desired output voltage
Linear regulators cannot provide an output voltage greater than the input voltage
VIN < VOUT
The first example of when to use a switching regulator – when you can not use a linear voltage regulator.
Linear voltage regulators are not capable of step-up (boost) operation. Therefore, if the minimum input voltage to your system is at or below the desired operating voltage you will need to use a switching voltage regulator.

11. Choosing Between Linear and Switching Regulators
When to use a switching regulator:
The efficiency of a linear regulator cannot maintain the junction temperature below the specified maximum
The maximum junction temperature is usually 150C
The efficiency of linear regulators often prohibit their use in high voltage, high current applications
As stated before, under most situations, linear regulators have a much lower efficiency than switching regulators. Low efficiency means a lot of power can be lost as heat. To make matters worse, linear regulators are lose efficieny as their input voltages (and power losses) increase. Sometimes even a giant heatsink will fail to keep the linear regulators junction temperature within the specified operating temperature range.
Under these conditions, the high conversion efficiency of a switching voltage regulator may be the only option.

12. Why are switching regulators needed?
The power dissipation is too high for a linear regulator
The efficiency of a linear regulator cannot maintain the junction temperature below maximum (150 °C)
The heat sinking of a linear regulator is prohibitive in price or space
Output Power
Switching Regulator
Linear Regulator
Maximum Power Dissipation
Linear Regulator

13. Why are switching regulators needed?
The desired output voltage is greater than the input voltage
Linear regulators cannot provide an output voltage greater than the input voltage
The desired output voltage is opposite polarity than the input voltage
Linear regulators cannot invert an input voltage
Power Supply
1.5 V
Battery
5 V
Required
Power Supply
12 V
Battery
-12 V
Required

14. Types of Switching Regulators AC-DC, AC-AC, DC-AC, and DC-DC Converters
110 Vac
110 Vac
12 Vdc
12 Vdc t t t t
220
Vac
Power supplies can be classified into a number of different categories. Two such categories are AC/DC converters and DC/DC converters.
In an AC/DC converter, the input power is delivered to the power supply as a true AC signal. The AC/DC power supply then creates a DC output voltage. We will not examine this type of power supply in this module.
Rather, we will address the DC/DC converter category of power supplies. With rare exception, DC/DC converters are used as the power supplies for automotive applications.
110 Vac
12 Vdc
5 Vdc t t t t

15. Types of DC-DC Converters Step Down, Step Up and Inverting
Vin = 12 V
Step Down
Buck
Vout = 5 V t t V V
Vout = 12 V
Step Up
Boost
Vin = 5 V t t
The DC/DC converter category of power supplies can be sub-divided by the ratio of input voltage to output voltage.
If the input power is delivered to the power supply at a voltage (VIN) which is less than the output voltage of the power supply (VOUT), the DC/DC converter must “boost” or “step-up” the voltage and the power supply is called a “Boost Converter” or a “Step-Up Converter”.
If the input power is delivered to the power supply at a voltage which is greater than the output voltage of the power supply (VOUT), the DC/DC converter must “step down” or “buck” the voltage and power supply is called a “Step-Down Converter” or a “Buck Converter”.
We will examine both buck and boost converters in this training module. V V
Inverting
Buck-Boost
Vin = 5V t
Vout = -10 V t

16. Basic Circuit Configuration
Buck
VIN > VOUT
Boost
VIN < VOUT
Buck-Boost
VIN < -VOUT < VIN
VIN
VIN
VIN
ISW
ISW
VGATE IL
VGATE IL L
VOUT
VOUT
VOUT L VM VM VM C C C
VGATE IL
ISW L
All topologies consists of the same basic components but are arranged differently

17. Buck Configuration
VIN
VIN
VOUT
ISW
20V
10V
VGATE
15V IL
7.5V
VOUT
10V 5V VM L 5V
2.5V C 0V 0V
time
time
The input voltage is always greater than the output voltage

18. Boost Configuration
VIN
VIN
VOUT
24V
20V
20V IL
15V L
15V
VOUT
10V VM
10V C 5V
VGATE 5V
ISW 0V 0V
time
time
The input voltage is always less than the output voltage

19. Buck-Boost Configuration
VIN
VOUT
VIN
time
ISW 0V
20V
VGATE
VOUT
15V
-5V VM
10V
-10V C IL 5V L
-15V 0V
-20V
time
The input voltage is always not constrained by the output voltage

20. Switched-Mode DC Power Supplies
Five Designing Topologies.
Flyback converter.
Forward converter.
Push-pull converter.
Half Bridge converter.
Full-Bridge converter.
Operate at high frequencies
Easy to filter out harmonics

21. Flyback Converter

22. Mode 1 Operation -- Q1 ON Current builds up in the primary winding
Secondary winding has the opposite polarity D1 OFF
C maintains the output voltage, supplies load current

23. Mode 2 Operation -- Q1 turned OFF
Polarity of the windings reverses
Diode D1 conducts, charging C and providing current to the load RL
Secondary current falls to 0 before the next cycle begins

24. Waveform Summary

25. Double-ended Flyback Converter

26. Forward Converter

27. Features Includes a “reset” winding to return energy.
Secondary “dot” so that D2 forward biased when Q1 is ON – no energy stored in the primary.
Operates in continuous mode.

28. Mode 1 Operation -- Q1 ON Current builds up in the primary winding
Energy transferred to the load

29. Mode 2 Operation -- Q1 turned OFF
Polarity of transformer voltages reverses
D2 turns OFF, D1 and D3 turn ON

30. Waveform Summary Vo D3

31. Double-ended Forward Converter

32. Push-Pull Converter

33. Push-Pull Operation Q1 ON, Vs across the lower primary winding
Q2 ON, Vs across the upper primary winding

34. Half-Bridge Converter

35. Mode 1 Operation
Q1 ON, D1 conducting
Energy transferred to the load

36. Mode 2 Operation Both transistors are OFF
D1 continues to conduct due to current in L1

37. Mode 3 Operation
Q2 ON, D2 conducting
Energy transferred to the load

38. Mode 4 Operation Both transistors OFF
D2 continues to conduct due to current in L1

39. Full-Bridge Converter

40. Mode 1 Operation Q1,Q4 ON, Q2,Q3 OFF
D1 conducting, energy transferred to the load

41. Mode 2 Operation All transistors are OFF
D1 continues to conduct due to current in L1

42. Mode 3 Operation Q2,Q3 are ON, Q1,Q4 OFF
D2 conducting, energy transferred to the load

43. Mode 4 Operation All transistors are OFF
D2 continues to conduct due to current in L1

44. How a Switching Regulator Works
VIN
VOUT
Switching Regulator 5V
Filter Network
Voltage OK
time
50%
Output
Monitor
VOUT
Duty Cycle Controller

45. How a Switching Regulator Works
VIN
VOUT
Voltage Regulator 5V
Filter Network
Voltage OK
time
50%
Duty Cycle Controller
Output
Monitor
VOUT

46. How a Switching Regulator Works
VIN
VOUT
Voltage Regulator 5V
Filter Network
Voltage OK
time
50%
Duty Cycle Controller
Output
Monitor
VOUT

47. How a Switching Regulator Works
VIN – 1V
VOUT
Voltage Regulator 5V
Filter Network
Voltage
Low
time
60%
Duty Cycle Controller
Output
Monitor
VOUT

48. How a Switching Regulator Works
VIN – 1V
VOUT
Voltage Regulator 5V
Filter Network
Voltage
Low
time
60%
Duty Cycle Controller
Output
Monitor
VOUT

49. How a Switching Regulator Works
VIN
VOUT
Switching Regulator 5V
Filter Network
Voltage Ok
time
50%
Output Monitor
VOUT
Duty Cycle Controller

50. Switching Regulator Components

51. Switching Power Supply Block Diagram
VIN
VOUT
Network
Switch
Network
PWM
Controller
Error
Amplifier
Bandgap
Reference

52. PWM Controller
In a switching voltage regulator, the pass transistor is used as a switch - it is either on or off
The output voltage, however, is an analog value
PWM controller senses error in VOUT via the error amplifier
PWM controller updates the duty cycle of the of transistor adjusting the output voltage
0-100%
Error
Amplifier
PWM
Controller
VOUT

53. Switching Transistor Bipolar and MOSFET
Switch Speed
Slow
Fast
Drive Method
Current
Voltage
Drive Circuit
Complex
Simple
ESD Robustness
High
Low
Drain
Collector
Base
Gate
Emitter
Source

54. Switching Power Supply Block Diagram
VIN
VOUT
Network
Switch
Network
PWM
Controller
Error
Amplifier
Bandgap
Reference

55. External Network
An external network (consisting of an inductor, capacitor, and diode) transforms the energy from the PWM controlled power switch into a desired output voltage
Switch
Network
VIN
VOUT
VIN = 12 V
VOUT = 5 V

56. Switching Power Supply Block Diagram
VIN
VOUT
Network
Switch
Network
PWM
Controller
Error
Amplifier
Bandgap
Reference

57. Step Down Switching Regulator Steady State Operation
VIN
VGATE
VGATE goes high
VM ~ VIN
VL = VM – VOUT t
ISW VM
VGATE IL t
-VF
VOUT
ISW VM
+ VL - t
RLOAD - VF + IL
COUT t
VOUT t

58. Step Down Switching Regulator Steady State Operation
VIN
VL Constant
VGATE t
ISW VM
IL and ISW increase
VGATE IL t
-VF
VOUT
ISW VM
+ VL - t
RLOAD - VF + IL
COUT is charged by IL
and
VOUT increases
COUT t
VOUT t

59. Step Down Switching Regulator Steady State Operation
VIN
VGATE = 0V
The pass transistor
is turned off
ISW = 0A
VGATE t
ISW VM
VM goes negative
VL = VM – VOUT
VGATE IL t
-VF
VOUT
ISW VM
+ VL - t
RLOAD - VF + IL
IL cannot go to
0A instantly:
COUT t
VOUT t

60. Step Down Switching Regulator Steady State Operation
VIN
But, VM is clamped
to -VF
and IL decays
through the diode
VGATE t
ISW VM
VGATE IL t
-VF
VOUT
ISW
VM = -VF
+ VL - t
RLOAD - VF + IL
COUT stabilizes
the output voltage
so VOUT will only slowly decay
COUT t
VOUT t

61. Step Down Switching Regulator Steady State Operation
VIN
The MOSFET is
turned on and off
to repeat
the sequence
VGATE t
ISW VM
VGATE IL t
-VF
VOUT
ISW
VM = -VF
+ VL - t
RLOAD - VF + IL
COUT t
VOUT t

62. REFERENCES For student study :
Read Chapter 21 discrete & integrated voltage regulators topics (21.5) from the book introductory electronic devices and circuits by (Robert T .paynter)conventional flow version.
Wikipedia & world wide web.

63. THANK YOU!